firewall: Add firewall API

firewall: Add firewall API

The main idea behind this API is to collect several iptables rules
together and enable or disable in one go.

For this a context is created via __connman_firewall_create() and
the rules added to this context via __connman_firewall_add_rule().

In order to append all rules __connman_firewall_enable() has
to be called. To remove all rules associated with one context
__connman_firewall_disable() has to be used.

If something goes awry the code tries to get back to the initial
state.